1. Field of the Invention
The present invention relates to a four-wheel steering apparatus for a vehicle, capable of steering front and rear wheels.
2. Description of the Related Art
In recent years, a four-wheel steering apparatus has become popular. With this apparatus, the rear wheels are steered in phase with the front wheels during high- and middle-speed traveling of a vehicle to cause the rear wheels to positively generate a cornering force, thereby improving driving stability during turning of the vehicle.
However, when in-phase steering control described above is performed, a yaw response at the start of turning of the vehicle is delayed to result in under-steering. In order to solve this problem, as disclosed in Published Unexamined Japanese Patent Application Nos. 59-186773 and 62-191272, there are known four-wheel steering apparatuses wherein rear wheels are temporarily steered in opposite phase to front wheels at the initial period of turning of the vehicle to assure a good yaw response of the vehicle and then the rear wheels are steered in phase with the front wheels to assure stability during turning of the vehicle. In each of these prior art four-wheel steering apparatuses, a rear-wheel steering mechanism is controlled in accordance with an output from a single steering control means. The rear wheels are temporarily steered in opposite phase to the front wheels at the start of steering, and then the phase is reversed to steer the rear wheels in phase with the front wheels.
When the above-mentioned phase reverse is performed by the single steering control means as in the conventional apparatuses, control is undesirably complicated, and it is difficult to sufficiently assure good response characteristics at the time of phase reverse.
The conventional four-wheel steering apparatus described in Published Unexamined Japanese Patent Application No. 59-186773 is designed to reverse the phase of the rear wheels when a predetermined period of time from the start of steering has elapsed. It is, however, difficult to set the predetermined period of time so as to satisfy both turning characteristics of the vehicle and convergence of the control.
The conventional four-wheel steering apparatus described in Published Unexamined Japanese Patent Application No. 62-191272 detects a turning operation and turning convergence to perform phase reverse of the rear wheels. However, when the steering wheel is returned, the phase of the rear wheels is reversed, and the vehicle behavior is abruptly changed. For this reason, the steering feel is spoiled, and passengers may feel uneasiness.